1. Field of the Invention
The invention relates to an apparatus and method for audio signal amplification and for the supply of power incorporated therein.
2. Background Art
Solid state circuit components have brought significant reduction in the size, weight and cost of audio amplifier circuitry and have also achieved increased fidelity and sound reproduction as compared with vacuum tube technology of a prior generation. In an attempt to exploit to the limit the potential of solid state circuitry, audio engineers have striven to provide amplifiers with increased power ratings while achieving high quality sound reproduction. One of the results of this has been that many commercially available state of the art high-powered amplifiers (e.g. 400 watts or more) typically weigh anywhere from 16 kg to as high as 30-40 kg, depending upon the particular design and choice of materials. A significant portion of this weight is in the transformer and the associated heat sinks.
A significant advance in the design of audio amplifiers and their associated power supplies is described in U.S. Pat. No. 4,484,150, the inventor being Robert W. Carver, one of the co-inventors in the present application. In the apparatus described in that patent, there is a power supply which directs a conventional, commerically available sinusoidally varying supply voltage to the primary winding of a power transformer. The secondary winding of the power transformer charges capacitor to a desired operating level, wih these capacitors in turn supplying the power to the amplifying apparatus itself.
The sinusoidally varying power supply directed to the primary is regulated in a manner that during periods of low power requirements, a switching means is closed at a later time in the latter half of each half-cycle of the sinusoidally varying input voltage to provide a series of low power pulses. However, during periods of higher power requirements, the timing of the switching is changed so that the voltage is imposed on the primary winding at an earlier time in the latter portion of each half-cycle. Thus, the current pulses delivered during periods of peak power requirements are of substantially greater power than those produced during periods of low power requirements. It was found that the apparatus of U.S. Pat. No. 4,484,150 permits a dramatic decrease in the size (and hence in weight) of the power transformer.
A significant consideration relative to audio amplifiers is that the audio signal varies greatly in amplitude, having a series of "peaks" and "valleys", with the time periods of the valleys generally being substantially greater than the time periods of the peaks. For example, even what it is considered to be consistently loud music (i.e. "hard rock") has such peaks and valleys. It was found that the power supply described in U.S. Pat. No. 4,484,150, when incorporated in an audio amplifier with the greatly varying power requirements, could be designed to be quite small and yet operate quite effectively and efficiently in that particular environment.
It has long been known in the prior art that the size of a transformer depends upon various factors, one of these being the frequency at which the transformer operates. As the operating frequency increases, the number of turns in the coils can be reduced correspondingly for the same amount of power output, and this in turn means that the core size also decreases. Thus, the prior art teaches audio amplifiers with a vrey high frequency power supply (e.g. 20,000 Hz), so that a very small transformer can be used. However, while these do have certain advantages, and while for certain applications and design objectives these may be desirable, there are accompanying design problems which offset to some extent the advantages derived from the very high frequency power supply.
Another consideration in this circuitry for the regulation of the switching for the power supply of an audio amplifier. To optimize the design of the amplifier, it is necessary, as indicated above, that the power being supplied is properly related to the greatly varying power requirements of the amplfier. In addition, the circuitry must be designed in view of other factors, such as the possibility of variation in the voltage derived from the power line, and other possible variations resulting from the operation of the amplifier itself. For example, if the line voltage is at a low level, the circuitry should be arranged to react properly to this situation and not react so as to overburden the power supply at a voltage level that would not be obtainable during that period of low line voltage.
It is within the foregoing considerations in mind that the apparatus in the method of the present invention were developed, with a primary object of the present invention being to provide an audio amplifier and method where the size and weight of the transformer can be made relatively quite small, and yet give the amplifier a preferred balance of desired operating characteristics relative to the factors noted above.